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Thread: Smithcub fuel system help needed

  1. #1

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    Smithcub fuel system help needed

    Hello all itís been a while since Iíve been here. My smithcub is now flying and on Amphib Clamarís. Iíve experienced the usual teething pains chasing squawks and have overcome most of the issues. Recently because of a leaky wing drain I emptied the right wing tank(24 gals) but was only able to get 17 gallons out thus identifying 7 gallons unusable. Obviously something is wrong and I suspect the rear pickup is not functioning and am seeking advice as how to diagnose the issue. Currently Iím draining the other tank to check itís unusable quantity.

    any help would be much appreciated

    thanks

  2. #2

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    Update

    The fuel selector was set to the off position and when I arrived to drain the left tank noticed the right tank filled itself and equalized itself with the left tank. Now I suspect the two tanks are somewhere connected. Very strange!

  3. #3
    stewartb's Avatar
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    Off means no fuel moves forward to the engine. The tanks can equalize in the off position, at least with valves I'm familiar with. If you want to isolate one tank select left or right. The tanks will not equalize with one tank selected.
    Last edited by stewartb; 08-02-2021 at 04:37 PM.
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  4. #4

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    "Unusable fuel" is not, according to FAA, the fuel left in the tanks after a static drain test in level flight attitude. "Unusable fuel" is properly determined by the worst case flight attitude that un-ports the tanks. The difference can be several gallons from that measured in a static level attitude test.
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  5. #5
    wireweinie's Avatar
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    What Stewart said. Do you know what fuel valve is installed? Maybe a pic of the valve? That would help for info on what the valve is manufactured to do.

    Web
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  6. #6

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    Quote Originally Posted by wireweinie View Post
    What Stewart said. Do you know what fuel valve is installed? Maybe a pic of the valve? That would help for info on what the valve is manufactured to do.

    Web

    The fuel valve is a 3 port weatherhead that was modified as per the installation instructions (Smithcub manual)

  7. #7

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    Quote Originally Posted by frequent_flyer View Post
    "Unusable fuel" is not, according to FAA, the fuel left in the tanks after a static drain test in level flight attitude. "Unusable fuel" is properly determined by the worst case flight attitude that un-ports the tanks. The difference can be several gallons from that measured in a static level attitude test.
    7 gallons left in the tank in a near level attitude is excessive in my opinion. I need to confirm the usable quantity and understand what happening with the system. I’m certain in a high angle of attack there would be even more unusable as it appears that only the front pickup is draining.

    There is no blockage at the rear line in the tank. I’m checking that the lines are not mixed up at the valve (not likely but possible). I don’t know what else could cause this condition.

  8. #8
    wireweinie's Avatar
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    The valve sounds normal. Are you running header tanks or no?

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  9. #9
    BC12D-4-85's Avatar
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    Just a curious guy here. This builder manual link (http://www.supercub.com/wp-content/u...ept2009-v5.pdf) might apply to the Smith Cub in question. Start at Page 62 for a description of the fuel system. If it's not applicable I'll delete the Post just let me know.

    Gary
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  10. #10
    wireweinie's Avatar
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    And remember that the front and rear outlets should join together in a tee fitting on the valve. Maybe a kinked line or obstruction in the line on the rear outlet?

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  11. #11

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    Sooooooo, a stock cub has right or left tank and off, no both. The tanks do not connect on the off. The mod described in the building instructions will have the tanks connected in the both and off mode. This is the same as the Dakota valve.
    DENNY

  12. #12

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    Pull the rear filter screen on the right tank, sounds like it is clogged. If it is the front will also have a bunch of trash in it. Then pull and clean every sump and drain in the system including the carb. Basically a cub should not have any unusable fuel.
    DENNY
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  13. #13

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    Quote Originally Posted by DENNY View Post
    Pull the rear filter screen on the right tank, sounds like it is clogged. If it is the front will also have a bunch of trash in it. Then pull and clean every sump and drain in the system including the carb. Basically a cub should not have any unusable fuel.
    DENNY
    The screen is clean in the right rear tank pickup and the rear lower drain has significant pressure and flows well. I drained about a gallon out to confirm. I'm now suspecting a blockage in the line between the lower rear drain sump and the fuel valve. I made the interior panels fit tight with little thought of having to remove them thus getting to the valve is going to be a bit of surgery. My attack plan is to change the fuel valve and blow out the line between the valve and the lower rear sump.
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  14. #14

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    Quote Originally Posted by DENNY View Post
    Basically a cub should not have any unusable fuel.DENNY
    Yet a CubCrafters FX-3 has 5 gallons "unusable" and takes a 30 lb hit on useful load. People who build an EX-3 and determine "unusable fuel" in a static level flight attitude test don't incur that hit.

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